1. Field of the Invention
The present invention relates to a heating roller of a hollow cylinder form, a heating device having the heating roller, a fixing device having the heating roller or the heating device, an image forming apparatus such as a copying machine, a facsimile machine, a printer, and a printing machine having the heating roller, the heating device or the fixing device, and a method of producing the heating roller.
2. Discussion of the Background
An image forming apparatus such as a copying machine, a facsimile machine, a printer and a printing machine generally includes a fixing device having a heating roller of a hollow cylinder form. The fixing device is a mechanism for fixing a toner image transferred onto a sheet.
The heating roller is made of metal in general, and is thinned for decreasing the heat capacity thereof so that heat response property thereof relative to a heating device which heats the heating roller can be improved. However, because of the thinning, the rigidity of the heating roller is decreased. When a heating roller 118 is used in a fixing device 116 illustrated in FIGS. 9, 10 and 11, a pressing roller 119 illustrated in FIGS. 9, 10 and 11 press-contacts the heating roller. Therefore, a bend is caused in the longitudinal direction of the heating roller 118 as illustrated in FIG. 9, or a crush is caused in the vertical direction of the shaft of the heating roller 118 as illustrated in FIG. 10. When the bend or the crush is caused in the heating roller 118, a gap is caused at a nip portion between the heating roller 118 and the pressing roller 119 as illustrated in FIG. 11, the bend or the crush becomes maximum in the center portion of the heating roller in the longitudinal direction thereof, and a nip width in a sheet-passing direction of the center portion becomes narrow. When the nip width thereof becomes narrow, a heat amount which is supplied to a transfer member, to which a toner image is transferred and which passes between the heating roller 118 and the pressing roller 119, becomes small, and a fixing failure occurs.
Such a failure becomes remarkable when the outside diameter of the heating roller 118 in the center portion in the longitudinal direction thereof is made smaller than that in both end portions in the longitudinal direction thereof in order to prevent wrinkles of the transfer member when the transfer member passes the nip portion. The failure becomes especially remarkable when the thickness of the heat roller 118 in the center portion thereof is made smaller than that in both end portions by cutting the outer circumference surface of the center portion.
In Japanese Laid-Open Patent Publication 10-228201, a technique has been proposed to perform material processing-hardening by adding an external force to the inner circumference surface and the outer circumference surface of a heating roller so that the strength of a heating roller is improved by the processing-hardening during material deformation. Further, in Japanese Laid-Open Patent Publication 10-039665, a technique has been proposed such that the strength of a heating roller is improved by forming a rib in the inner circumference surface of the heating roller.
However, an apparatus using the technique proposed in Japanese Laid-Open Patent Publication 10-228201 is large and has low productivity. Moreover, in the technique in Japanese Laid-Open Patent Publication 10-039665, the heat capacity of a heating roller is increased in the portion thereof in which the rib is formed, and thereby it is difficult to maintain a uniform temperature of the heating roller as the temperature is rising.